Lignocellulosic biomass is plant biomass that includes cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are carbohydrate polymers that are tightly bound to the lignin. Lignocellulosic biomass can be grouped into four main categories: (1) agricultural residues; (2) energy crops; (3) wood residues, including sawmill and paper mill discards; and (4) municipal paper waste. Lignocellulosic biomass represents a potentially sustainable source of fuel and commodity chemicals, offers economic advantages over corn starch for the production of biofuels, and could contribute to carbon sequestration without impacting food crop prices. Lignocellulosic biomass could satisfy the energy needs for transportation and electricity generation, while contributing to carbon sequestration and limiting the accumulation of greenhouse gases in the atmosphere.
Potential feedstocks of lignocellulosic biomass are abundant and include crops (e.g. corn and sugarcane), agricultural wastes, forest products (e.g. wood), grasses, and algae. Among the feedstocks, wood has been widely used for the production of paper, as a construction material, and as a solid fuel. Wood is composed mainly of cellulose, hemicellulose, and lignin. Lignin includes an amorphous network of crosslinked phenylpropanoid units.
The conversion of lignocellulosic biomass into liquid fuels and/or other commodity chemicals typically includes the following steps: (1) pretreatment; (2) hydrolysis of cellulose and hemicellulose into fermentable sugars; and (3) fermentation of the sugars into the liquid fuels (e.g. ethanol) and other commodity chemicals. The pretreatment is energy-intensive but necessary due to the complex structure of the plant cell wall and the chemical resistance of lignin, which limits the access of enzymes to cellulose. An ideal pretreatment should break the lignocellulosic complex, increase the active surface area, and decrease the cellulosic crystallinity while limiting the generation of inhibitory by-products and minimizing hazardous wastes and wastewater.
A major bottleneck in the large-scale conversion of biomass to biofuels is the pretreatment delignification process that provides enzymes access to cellulose, the main source of fermentable sugars. Most current pretreatments, such as ammonia fiber explosion, alkaline hydrolysis, and acid hydrolysis, require high temperatures that increase the operation costs and generate toxic byproducts. The pretreatment is also the most expensive step in the conversion of lignocellulosic biomass to ethanol. Less expensive pretreatments that are environmentally friendly are desirable.
Various treatments have been investigated for removing the lignin, which would improve the yield of fermentable sugars for the production of biofuels. Most of these treatments are energy-intensive and/or generate toxic byproducts that affect their economic viability. A “green” pretreatment strategy was reported to replicate and optimize the enzymatic activity of natural white rot fungi such as Phanerochaete chrysosporium, which degrade biomass completely, including the lignin. The reactivity of the manganese-dependent peroxidase enzyme produced from P. chrysosporium was studied with lignin model compounds. The enzyme requires MnII and H2O2 to degrade lignin. The reaction is proposed to involve the oxidation of MnII to MnIII, a known one-electron oxidant. Stoichiometric Mn(OAc)3 was shown to oxidize guaiacyl and syringyl lignin models at a pH ranging from 2.5 to 4.1 at room temperature. The use of Mn(OAc)3 as a catalyst for the delignification of wood with H2O2 has not been reported.
Studies involving the use of transition metal catalysts and H2O2 for the oxidative delignification of wood have been reported. A study related to the ability of several metalloporphyrins based on Fe, Co, Zn and Mn to oxidize lignin models has been reported. This work has been limited by the complex nature of these metalloporphyrins and their costly synthesis, as well as difficulties associated with catalyst recovery. Several other Mn and Fe complexes of synthetic macrocyclic ligands, including [(TACN)Mn(μ-O)3Mn(TACN)](PF6)2 (TACN=1,4,7-trimethyl-1,4,7-triazacyclo-nonane) and polypyridyl ligands, were shown to catalyze the delignification of wood using hydrogen peroxide. Methyl-trioxorhenium was reported to catalyze the oxidation of lignin model compounds and lignin. Although promising activity was observed at 60° C. using [(TACN)Mn(μ-O)3Mn(TACN)](PF6)2, the expensive synthetic ligands have limited the application of this type of catalyst.
A combination of Co(OAc)2 and Mn(OAc)2 was reported to catalyze the aerobic auto-oxidation of lignin in acetic acid solution at 140-220° C. In ionic liquid solution, Mn(NO3)2 and CoCl2 were found to catalyze the oxidation of various lignin and lignin models in air, but this also requires elevated temperatures (80-100° C.) and/or high pressures (83 atm).
A simpler process for delignification of lignocellulosic biomass may improve the economic viability of oxidative delignification.